Acts 11 records an interesting moment of generosity for something that hadn't even happened yet. A prophet predicts a famine and fairly new believers in Antioch give money to Barnabas and Paul for the church in Jerusalem. Josephus records this famine that happened was in AD 45-48.

When you read the text it goes something like this; the profit speaks and the people gave according to their means. I don't think it was as simple as that. What I think happened was under the teaching of Barnabas and Paul who both had a revelation and heart for generosity, the new believers in Antioch were inspired to give.

In a stunning act of generosity, Barnabas sold some land and gave the proceeds to the early forming church in Jerusalem. Paul, a hard-working Apostle continually encouraged generosity on his travels around the Roman empire.

If you are a church leader who struggles to teach about generosity, chances you are personally struggling with it. You can't credibly teach what you don't believe. When asked about teaching generosity from pastors I ask this question, do you have a revelation of biblical generosity? If you don't trust God with your money, how can you expect your congregation to do so?
Paul and Barnabus' revelation on giving so affected new believers, they gave to famine-affected believers before the famine even happened.

This particular offering was voluntary but prompted by Spirit-let leaders, it was in proportion to the giver's means i.e. all the amounts were different and everyone participated.
It's a pretty good outline for taking offerings in the 1st century and the 21st.